There is a continuing need for compact, efficient and less expensive electrical machines that have high torque capability over a large speed range and the ability to control machine speed. This need is particularly significant for electrical drives for vehicles, such as hybrid automobiles, which require high torque at zero and low speed, fast acceleration, and preferably regenerative braking. Permanent magnet machines have drawn increasing attention for their potentially greater capability than conventional AC and DC motors to meet these stringent requirements.
The development of permanent magnet technology, such as the availability of more powerful NdFeB magnets, has resulted in increased power and torque density and greater efficiency for permanent magnet machines generally. Permanent magnet machines are usually more efficient because field excitation losses are eliminated. In addition, copper losses are generally reduced in permanent magnet machines as compared to conventional machines. Due to lower losses, heating in the permanent magnet machines should be less than in conventional machines, which allows either operation of the machine at lower temperatures or an increase in shaft power until the maximum allowable temperature has been reached. Typically, less power is required from the power electronics converter in order to deliver the same power to a permanent magnet machine because of the high efficiency of such machines.
Air gap flux control of permanent magnet machines can generally be accomplished either by control techniques or a suitable modification of the machine structure. Conventional permanent magnet machines have a fixed magnetic excitation, which limits the capability of the power electronic drive. Such machines are operated at constant volt-hertz operation up to a base speed; constant voltage operation at higher speeds requires weakening of the field in order to extend the speed range. Above base speed, vector control techniques are typically used to weaken the air-gap flux. However, these techniques cause large demagnetization current to flow in the machine d-axis, resulting in higher losses and the risk of demagnetization of the permanent magnets. The magnets may ultimately be forced to operate in the irreversible demagnetization region. See, T. M. Jahns, IEEE Trans. on Ind. App., Vol. 23 No. 4, July-August 1987, pp. 681-689; T. Sebastain and G. R. Slemon, IEEE Trans. on Ind. App., Vol. 23 No. 2, March-April 1987, pp. 327-333. Such demagnetization can permanently diminish the torque capacity of the machine. Thus, the attainable speed range is limited by the largest tolerable demagnetization current. In addition, the capability of the electrical power converters sets an additional limit to the flux weakening range of the permanent magnet machine.
Efforts have been made to realize field weakening in permanent magnet machines by eliminating the detrimental effects of d-axis current injection. A number of alternative solutions have been proposed. These include a double salient permanent magnet machine with flux control as shown in U.S. Pat. No. 5,455,473; a double salient permanent magnet machine capable of field weakening, (A. Shakal, et al., IEEE Int. Sym. on Ind. Elect. Conf. Proc., 1993, pp. 19-24); an outer rotor double salient permanent magnet machine (J. Luo, PhD thesis, University of Wisconsin-Madison, 1999); a two-part rotor synchronous permanent magnet machine (B. J. Chalmers, et al., IEEE Proceedings, Vol. 145, No. 2, March 1998, pp. 133-139); a radial flux permanent magnet machine adapted for air gap flux weakening operation (L. Xu, et al., IEEE Trans. on Ind. App., Vol. 31, No. 2, March-April 1995, pp. 373-378); a consequent pole permanent magnet machine (J. A. Tapia, et al., IEEE Int. Conf. on Elect. Machines and Drives, Boston, 2001, pp. 126-131);a hybrid electric machine as shown in U.S. Pat. Nos. 6,462,430, 6,541,877, and 5,777,022; a brushless motor with permanent magnets as shown in U.S. Pat. No. 5,821,710; and a brushless permanent magnet machine with variable axial rotor-stator alignment to increase speed capability as shown in published International Application WO 03077403 A1, 2003.
Axial flux permanent magnet machines have drawn increasing attention within the last decade. They provide certain advantages over conventional radial flux permanent magnet machines, including higher power-torque density and efficiency, more easily adjustable air gaps, and lower noise and vibration levels. Axial flux machines can have a variable air gap which may be suitable for some flux weakening applications such as electric traction. An axial flux machine composed of two slotted stators and a single rotor is discussed in F. Profumo, et al., IEEE Ind. App. Soc. Annual Meeting 1998, pp. 152-158. The slotted side of the stator has a tape wound core with series connected stator windings. The rotor structure has an axially magnetized rotor disk having main and leakage poles. There are two flux barriers between the leakage and main poles. Another axial flux machine with flux control is discussed in U.S. Pat. No. 6,057,622, and in J. S. Hsu, IEEE Trans. on Energy Conversion, Vol. 15, No. 4, December 2000, pp. 361-365. This machine uses a field weakening coil to achieve field weakening by directly controlling the magnitude and polarity of a DC current in the field weakening coil. The rotor is formed by magnet and iron pole pieces which are mounted in holes in a non-magnetic rotor body. The machine has two slotted stators and AC windings, and each stator has a yoke providing a flux return path. Two field weakening coils in toroidal form are mounted on a machine frame. Another axial permanent magnet machine with flux control is described in U.S. Pat. No. 6,373,162. This machine includes two stators and one rotor, with the rotor having permanent magnets and pole portions. The magnets in the rotor generate a first magnetic flux and the consequent rotor poles generate a second magnetic flux. A field coil, which is mounted to the housing and located very close to the rotor, is effective to vary the second magnet flux. A brushless axial flux permanent magnet synchronous alternator is described in N. L. Brown and L. Haydock, IEE Proc. of Elec. Power Appl., Vol. 150, No. 6, November 2003, pp. 629-635.
Flux control capability is an important objective for permanent magnet machines. Unfortunately, surface mounted permanent magnet machines, by their nature, lack the capability to control the flux. What is needed is a surface mounted permanent magnet machine that has the capability to control the flux without over-sizing the machine. What is further needed is a surface mounted permanent magnet machine that has the capability to control the flux in a cost effective manner.